This works beautifully for a few minutes, then exits normally according to the logs. I think the firewall closes ssh connections with no traffic after 60 seconds.

So how do I enable the KeepAlive -K parameter on dropbear? If I tack it on this statement, autossh says its incorrect syntax. Also, the -o "ServerAliveInterval 30" -o "ServerAliveCountMax 999", while good for autossh, aren't used by dropbear.

That's the problem w/ dropbear; it's not a full SSH implementation. That's why it helps to have a router capable of installing Optware, where you can install full-featured packages (e.g., OpenSSH). Dropbear has plenty of other limitations too (e.g., no dynamic proxy (-D), proprietary private/public key format (ugh)).